1. Field of the Invention
The present invention relates to pipe connectors for connecting members, such for example as pipes particularly but not exclusively for use in conveying pressurized fluids for example in oil and gas exploration and production.
2. Description of the Prior Art
The main type of pipe connector presently used in oil and gas exploration and production for connecting tubings and casings comprises a female member having an internal frusto-conical circumferential surface provided with a thread of constant pitch, and a male member having an external correspondingly frusto-conical circumferential surface and provided with a thread having the same pitch as the thread of the female member and for threaded engagement with the female member. Sealing of the connector is obtained by causing a generally axially directed sealing surface on the free end of the male member to abut against a corresponding sealing surface of the female member under an axial compressive force. This force is obtained by screwing the members together tightly causing compression of the male member and tensioning of the female member and is usually maintained by providing an interference fit between crest and root surfaces of the threads.
Pipe connectors of this type have been found to have a number of substantial disadvantages. The axial compressive stress in the male member and the associated tensile stress in the female member required to force the sealing surfaces into sealing contact is borne predominantly by only the last few turns of the thread adjacent the sealing surfaces, the remainder of the turns of the thread being at most very lightly loaded. Consequently the load bearing leading flanks of the loaded turns of the thread experience substantial loads while sliding relative to one another to bring the sealing surfaces into compressive contact. Additionally, while increasing the radial interference between the threads of the members will reduce the tendency of the members to unscrew or back-off, the resulting hoop stresses in the female member, particularly when increased under the influence of internal pressure, can exceed the stresses at which the material of the member will yield. Furthermore high hoop stresses and high flank loading can produce galling of the thread surfaces.
The relatively short axial distance between the load bearing turns of the threads and the sealing surfaces limits the strain energy induced in the members so that a slight relative axial movement of the members of the connector can significantly reduce the integrity of the seal between the sealing surfaces. Not only is the strain energy limited but also the angle through which the members are rotated in producing the sealing force on the sealing surfaces is limited so that any slight tendency of the members to unscrew or back-off, for example resulting from a lateral blow to one of the members, will cause a significant reduction in the compressive loading of the sealing surfaces.